It is known from the present applicant's earlier international patent application published under No. WO 03/058061, to provide a vertical axis wind turbine with three blades or sails spaced at an angle of substantially 120° which is able to provide a substantially constant torque output. In particular, the leading surface of the sails develops lift (in a manner similar to the headsail of a yacht) whilst the trailing surface of the blades develops drag (in the manner of the mainsail of a yacht sailing before the wind).
The present invention also relates to turbines including the Savonius turbine, the vertical axis turbine of WO 03/058061, and to the vertical axis wind turbine disclosed in Australian Patent Application No. 2004 906 051 (lodged 20 Oct. 2004 and presently at the priority date of the present application). In the two last mentioned specifications, the turbine has one or more stages and each stage has preferably three blades each of which functions in a manner somewhat akin to a sail so that an aerodynamic force is generated by the blades which rotates the turbine, irrespective of whether the blades are moving in the direction from which the wind is blowing or are moving into the direction from which the wind is blowing. This is explained in more detail in the abovementioned PCT specification.
In general, the larger the radial extent of the blades of the turbines, the greater the power able to be generated by the turbine. However, as the radial extent of the blades increases, so do the mechanical loads on the blades, the cost of construction, and like factors.